The present invention is directed to a brushless DC motor system, and more particularly to a motor system in which commutation is based upon an optically scannable member which is rotated by the motor. The invention is particularly useful for driving a head drum in a video tape recorder.
In prior art electronically commutating, multipole DC motors for driving record players, tape recorders, and video recorders, the coils are commutated as a function of signals emitted, for example, by Hall effect elements. The Hall effect elements are arranged stationarily in a certain association with the coils. The rotor is provided with the permanent magnet, and the Hall effect elements detect the polarity and magnitude of the field lines emitted by the permanent magnets. Depending on the structure of the motor, several Hall effect elements are required. Although Hall effect elements are able to detect polarity, they are too imprecise for phase regulation as required, for example, in video recorders.
Moreover, tachogenerators or frequency generators coupled to the motor shaft are required to regulate the number of revolutions, with their signals being evaluated in a servo system for control. Motors provided to drive a head drum in a video recorder additionally require a so-called pick-up pulse for phase control, i.e., for the association of the moment of magnetic head switching with the sync pulses of the video signal. This pick-up pulse must be generated at a fixedly given position of the head drum and thus also of the motor.
Circuits are also known for detecting the moment of commutation and the phase position. These circuits employ inductive magnetic field sensors. Hall effect elements as well as magnetic field sensors are relatively expensive and their detection capability at low motor rpm is relatively poor. Moreover, the signal amplitude emitted by these sensors is relatively low, so that much amplification is subsequently required for the evaluation.